I'm wondering if you have access to a resource that could help explain a technical issue that seems to have a number of us confused. Some argue that a 1.6x camera (ex: EOS 40D) will have more depth of field than a FF camera (ex: EOS 5D) when used with the same lens at the same focal length with the same camera to subject distance at the same aperture (as I recall, the three factors that affect DoF). Most of these folks base their statements on experimentation. Others contend that with all factors the same, other than sensor size, the DoF will be the same, but you will see less picture on the 1.6x camera as compared to the FF image. Most of these folks base their answer on "science." In order to get the same composition with the 1.6x camera, one has to move further back from the subject, and refocus, which would (understandably) result in more depth of field. If you have an authority on this subject that could post in our ongoing discussion, I'm sure it would be greatly appreciated. Perhaps there is some other factor that we have not considered that results in more DoF.

Here is my understanding on this topic: The three main factors (not the only factors, but the main factors) affecting depth of field are camera-to-subject distance, focal length, and aperture. Changing any of these will have an effect on the depth of field in the resulting photograph. When shooting the same scene with both full frame and small sensor digital SLRs, typically there are two ways that photographers attempt to match the angle of view:

1. If the actual focal length is the same on both cameras, the camera with the smaller sensor must be positioned at a further camera-to-subject distance to match the angle of view. Increasing the distance while keeping the focal length and aperture the same results in greater depth of field.

2. If the distance and the aperture remain the same for both cameras, the actual focal length must be reduced on the camera with the smaller sensor to match the angle of view on the full-frame camera. Reducing the actual focal length without changing the distance and aperture also results in greater depth of field. It turns out that there is approximately a one f/stop increase in depth of field for a 1.6x DSLR compared to a full-frame DSLR when the distance and aperture are the same, but the focal length is reduced on the 1.6x camera to match the angle of view on the full-frame camera.

3. If all three factors are the same on both cameras, then depth of field is identical, but the angles of view are not.

What may be confusing some photographers on this topic is the concept of "effective" focal length vs. actual focal length. For example, it is often said that an actual focal length of 50mm on a 1.6x DSLR is equivalent to an "effective" focal length of 80mm on a full-frame DSLR. While this may be true in terms of angle of view, it is equally true that the actual focal length of the lens never changes regardless of image sensor size. When it comes to comparing depth of field, therefore, "effective" focal lengths should be disregarded.

One wish list item for the firmware on the EOS-1D Mark III (and 1Ds Mark III when it's here) would be for Canon to allow us to enable or disable the various drives modes. Given that almost every other function on the camera can be enabled or disabled – e.g., shooting modes, metering modes, etc. – it would make sense to allow the user to disable drive modes which are not required. As a wedding shooter I only ever use S (Silent/Delayed mirror return), single shot and low speed drive mode. High speed drive mode is just embarrassing if it is accidentally selected (the icons are a little small and indistinct for my fading eyes) and I'm never going to use either of the self timer modes during a wedding …

I'll be happy to pass along your suggestion, but in the meantime there is a workaround that may be helpful for you: C.Fn III-16 lets photographers adjust the framing rate for high speed continuous from 2 to 10 frames per second. Low speed continuous can be adjusted from 1 to 9 frames per second. Taking advantage of this feature by setting a lower framing rate than the defaults may save you a few headaches if you set the wrong drive mode accidentally.

I'm noticing a significant difference when comparing color reproduction of the same scene on the LCD screen of my new EOS-1D Mark III versus my EOS 5D. What's going on?

There are many different aspects to consider when comparing the LCD displays of the EOS-1D Mark III to other EOS bodies, especially the 5D. First of all, there have been two versions of the 5D LCD. Early production models have a different backlight than current production models. The newer version is warmer than the older one, but that's not why it was changed. The main reason was to increase the brightness level to make the screen easier to see in daylight conditions. If you compare an early production 5D to a current production model, the current one is a full two steps brighter. At the same time, the newer version is warmer, as previously mentioned. Once you get used to either one, it's difficult to feel comfortable with the other.

The 3-inch LCD for the EOS-1D Mark III is even brighter than the current version of the 5D. It uses the same kind of backlight illumination, so the color balance is similar, but there are more LEDs so the display is brighter, at least when both screens are set to their defaults. Under these circumstances, any fair comparison of the screens for 5D vs. 1D Mark III has to take into account which version of the 5D you're comparing. If it's an early production model, it's going to look cooler overall, tending towards greenish if you compare the same image on both. If it's a current production model, it's going to be similar to the 1D Mark III, but smaller of course. But that's just the tip of the iceberg. Zeroing in on differences when photographing under tungsten illumination, there are at least three more issues to confront:

1. Picture Style: Whether you shoot RAW or JPEG, the overall color balance shown on the LCD display is going to be affected to a noticeable degree by your choice of Picture Style setting on the camera's menu. If you use the default Standard setting, you're going to find that the colors in the display tend to be well saturated, perhaps too well saturated for some photographers' tastes. Of course it's possible to override all color balance settings in post-processing with RAW images, but when it comes to judging color on the camera's LCD display, this is something you need to be aware of. If you haven't tried it yet, I would encourage you to compare the Neutral Picture Style to Standard when you're shooting under tungsten illumination. Keep in mind that you can freely adjust Sharpening, Contrast, Saturation and Color Tone in any Picture Style.

2. Light Source Color Temperature: Photographers tend to talk about tungsten illumination as if it were fairly standardized, but obviously it is not. Significant variations can be caused by a number of factors, including the voltage of the lamps and the coloration of lamp shades, among other things. Resulting color temperatures can often range from 3600K or 3800K down to less than 2000K, with huge differences in color balance as a result.

3. Camera's White Balance Setting: Several photographers have commented that the EOS system's handling of AWB under tungsten illumination is less than adequate, and of course everyone is entitled to an opinion. However, it should be pointed out that, as listed in the instruction manuals, Canon's AWB does not adjust to WB settings under 3000K. If the light source is less than 3000K, what do you get? Warm photos, what a surprise! (Not). Setting the WB to Tungsten can be helpful, but you'll get even more control by using the manual color temperature setting (the K setting) or custom white balance. The EOS-1D Mark III has an edge over the 5D here, because (a) it can be manually set to 2500K instead of just 2800K, and (b) it can register up to 5 CWB readings instead of just one.

To summarize, if you want to make a suggestion that EOS AWB should extend its correction range to less than 3000K, that's perfectly understandable, and I'll gladly pass it along to our R&D folks. Similarly, if you feel that the 1D Mark III's LCD display is too warm in color balance for your taste, that's another opinion that I am happy to relay. But for those of you who want to make the best of what you've got, please consider the suggestions I've made concerning choice of Picture Style and White Balance settings. If you try them out, you will see a difference, and it may make the LCD screens easier to use when evaluating color on location.

Thanks for reading Tech Tips! That's it for now. See you in November!

You are invited to submit questions about photo equipment, imaging technology, or photo industry trends that may have a bearing on your work or interests. I cannot promise to answer everything, but I pledge to do my best to address the issues that concern you. (Please use the e-mail link provided at the end of this article.)

After earning a degree in Professional Photography from the Rochester Institute of Technology and accumulating some valuable on-the-job experience during a 10-year stint in commercial photography and photo retail, Chuck Westfall began his corporate career with Canon U.S.A. in 1982 as a Technical Representative. He has steadily advanced through the ranks to achieve his present position as Director of Media & Customer Relationship for the company's Consumer Imaging Group, working out of Canon U.S.A.'s headquarters office in Lake Success, N.Y. Among his many assignments, Chuck Westfall is currently Canon USA's main media spokesman for new camera products. He also provides a unique insider's perspective to financial analysts who follow the company's CIG sales and marketing activities.

Chuck's involvement with digital cameras began in 1994, when he assisted Canon and Kodak engineers in developing the EOS-DCS series of professional SLRs. Since then, his responsibilities have expanded to include participation in the development and launching of many other Camera Division products, including Canon's professional and consumer-oriented digital cameras. Over the last 10 years, Chuck has continued to participate in the design, development, introduction and marketing support of camera products. Most recently, he supervised the launch of a comprehensive on-line and on-site dealer training initiative for the Camera Division.

On the personal side, Chuck married his beautiful wife Ying in 2000 and they have been blessed with a wonderful daughter, Anna. As Chuck says, "Bringing up the baby is a blast, and we're enjoying every minute of it."